The graph of the function $f$, where $y=f(x)$, gives the total monthly cost $y$, in dollars, for a cell phone plan that includes a base fee and a charge per gigabyte of data used, where $x$ is the number of gigabytes used. The graph shows a line that passes through the points $(0,30)$ and $(10,70)$. What is the total monthly cost, in dollars, if a user uses 15 gigabytes of data?